Sulfur hexafluoride (SF6) has been used as a gaseous dielectric (insulator) in high voltage equipment since the 1950s. It is now known that SF6 is a potent greenhouse warming gas with one of the highest global warming potentials (GWP) known. Because of its high GWP, it is being phased out of all frivolous applications. However, there is currently no known substitute for SF6 in high voltage equipment. The electrical industry has taken steps to reduce the leak rates of equipment, monitor usage, increase recycling, and reduce emissions to the atmosphere. However, it would still be advantageous to find a substitute for SF6 in electrical dielectric applications.
The basic physical and chemical properties of SF6, its behavior in various types of gas discharges, and its uses by the electric power industry have been broadly investigated.
In its normal state, SF6 is chemically inert, non-toxic, non-flammable, non-explosive, and thermally stable (it does not decompose in the gas phase at temperatures less than 500° C.). SF6 exhibits many properties that make it suitable for equipment utilized in the transmission and distribution of electric power. It is a strong electronegative (electron attaching) gas both at room temperature and at temperatures well above ambient, which principally accounts for its high dielectric strength and good arc-interruption properties. The breakdown voltage of SF6 is nearly three times higher than air at atmospheric pressure. Furthermore, it has good heat transfer properties and it readily reforms itself when dissociated under high gas-pressure conditions in an electrical discharge or an arc (i.e., it has a fast recovery and it is self-healing). Most of its stable decomposition byproducts do not significantly degrade its dielectric strength and are removable by filtering. It produces no polymerization, carbon, or other conductive deposits during arcing, and its is chemically compatible with most solid insulating and conducting materials used in electrical equipment at temperatures up to about 200° C.
Besides it good insulating and heat transfer properties, SF6 has a relatively high pressure when contained at room temperature. The pressure required to liquefy SF6 at 21° C. is about 2100 kPa; its boiling point is reasonably low, −63.8° C., which allows pressures of 400 kPa to 600 kPa (4 to 6 atmospheres) to be employed in SF6-insulated equipment. It is easily liquefied under pressure at room temperature allowing for compact storage in gas cylinders. It presents no handling problems, is readily available, and reasonably inexpensive.
SF6 replaced air as a dielectric in gas insulated equipment based on characteristics such as insulation ability, boiling point, compressibility, chemical stability and non-toxicity. They have found that pure SF6, or SF6-nitrogen mixtures are the best gases to date.
However, SF6 has some undesirable properties: it can form highly toxic and corrosive compounds when subjected to electrical discharges (e.g., S2F10, SOF2); non-polar contaminants (e.g., air, CF4) are not easily removed from it; its breakdown voltage is sensitive to water vapor, conducting particles, and conductor surface roughness; and it exhibits non-ideal gas behavior at the lowest temperatures that can be encountered in the environment, i.e., in cold climatic conditions (about −50° C.), SF6 becomes partially liquefied at normal operating pressures (400 kPa to 500 kPa). SF6 is also an efficient infrared (IR) absorber and due to its chemical inertness, is not rapidly removed from the earth's atmosphere. Both of these latter properties make SF6 a potent greenhouse gas, although due to its chemical inertness (and the absence of chlorine and bromine atoms in the SF6 molecule) it is benign with regard to stratospheric ozone depletion.
That is, greenhouse gases are atmospheric gases which absorb a portion of the infrared radiation emitted by the earth and return it to earth by emitting it back. Potent greenhouse gases have strong infrared absorption in the wavelength range from approximately 7 μm to 13 μm. They occur both naturally in the environment (e.g., H2O, CO2, CH4, N2O) and as man-made gases that may be released (e.g., SF6; perfluorinated compound (PFC); combustion products such as CO2, nitrogen, and sulfur oxides). The effective trapping of long-wavelength infrared radiation from the earth by the naturally occurring greenhouse gases, and its reradiation back to earth, results in an increase of the average temperature of the earth's surface. Mans impact on climate change is an environmental issue that has prompted the implementation of the Kyoto Protocol regulating the emissions of man made greenhouse gases in a number of countries.
SF6 is an efficient absorber of infrared radiation, particularly at wavelengths near 10.5 μm. Additionally, unlike most other naturally occurring green house gases (e.g., CO2, CH4), SF6 is only slowly decomposed; therefore its contribution to global warming is expected to be cumulative and long lasting. The strong infrared absorption of SF6 and its long lifetime in the environment are the reasons for its extremely high global warming potential which for a 100-year time horizon is estimated to be approximately 22,200 times greater (per unit mass) than that of CO2, the predominant contributor to the greenhouse effect. The concern about the presence of SF6 in the environment derives exclusively from this very high value of its potency as a greenhouse gas.
Accordingly, many in the electrical equipment industry have spent substantial time and effort seeking suitable replacement gases to reduce the use of SF6 in high voltage electrical equipment. To date, the possible replacement gases have been identified as (i) mixtures of SF6 and nitrogen for which a large amount of research results are available; (ii) gases and mixtures (e.g., pure nitrogen, low concentrations of SF6 in N2, and SF6—He mixtures) for which a smaller yet significant amount of data is available; and (iii) potential gases for which little experimental data is available.
Some replacements which have been proposed have higher GWPs than SF6. For example, CF3SF5 falls into this category. Because of fugitive emissions in the manufacture, transportation, filling and use of such chemicals, they should be avoided.
However, the present inventors have determined that given the environmental difficulty of SF6, it is necessary to relax certain of the requirements traditionally held as important and accept as an alternative gas, compromise candidates with a lower GWP. For example, gases which are non-toxic are often inert with long atmospheric lifetimes which can yield high GWP. By accepting a somewhat more reactive gas than SF6, the GWP can be greatly reduced. It may also be necessary to accept slightly more toxic materials in order to find the best alternative in these applications. Such an increase in toxicity can be offset by reducing equipment leak rates or installing monitoring equipment. In some cases, the gases discovered by the present inventors as suitable alternatives to SF6 are show to be efficient at low levels and can be mixed with nitrogen and/or another non-toxic gas to give dielectrics with greatly reduced toxicity and acceptably low GWPs.
The unique gaseous compounds discovered by the present inventors for use as substitutes for SF6 can be used in some existing electrical equipment, although they would preferably be used in specific electrical equipment optimized for them. The gaseous compounds of the present disclosure are preferably used in pure form, but can also be used as part of an azeotrope, or a mixture with an appropriate second gas, such as nitrogen, CO2 or N2O.